The changes in electrical impedance (EI), which is related to the size of extracellular spaces, were studied in cats subjected to the left MCA occlusion, either permanent or for one hour. The main findings indicate that shortly following MCA occlusion there is a prompt increase of EI due to reduction of extracellular spaces produced by intracellular uptake of interstitial fluid. The release of one hour occlusion is followed by a drop in EI which is due to temporary recovery of the cells and restitution of osmostic balance. The progressive rise in intracranial presssure induces a secondary rise in EI which then after 12-24 hours is followed by a drastic drop in EI signifying enlargement of extracellular compartment produced by ruptures of cell membranes. These studies allow monitoring of basic pathophysiological events occurring during and after an ischemic insult. This may provide an indication for timing various therapeutic approaches which might be different in various stages of an ischemic lesion.